Fluid Power World contributing editor, Carl Dyke, founder of CD Industrial Group, has related a number of field-service issues he’s come across over the years. One was discussed in the recent post, “Troubleshooting tales from the trenches.” Here’s another real-world application, from Carl’s presentation at the 2017 Fluid Power Technical Conference in Milwaukee.
This one was a lot of fun. It involved a 360-ton haul truck, and a telescopic cylinder that was being taken off the machine for repair. A quick glance showed a tiny bit of silver color from the second stage barrel, which isn’t fully retracted, and the body of this truck not sitting on the chassis rails. And the problem in this case: something had caused the second stage barrel, with the steel being about three-quarters to maybe even an inch thick, to actually swell. What type of pressures would we be talking about to hydroform a piece of steel that thick?
Well, the site managers had gone through this a number of times with their fleet of trucks over a period of two years, and the truck manufacturer had brought in data acquisition recording devices, put pressure transducers in the system, and had logged some interesting data—but none of it was all that conclusive.
Most of the parties involved wanted to blame the hoisting valve. But the curious part for us, as we got involved, was that the hoisting valve has good old flexible hydraulic hoses running out to the cylinders. And if the valve was the cause of the system to hydroform the second stage barrel in a telescope like this, we asked, “How many hose blowout incidents have you had, where you’ve blown a hose or popped a hose out of a crimp fitting, over the same two years?” They answered, “Well, we haven’t had any.” Okay, that’s good to know.
So we got into it a little bit, and noticed that the counterbalance valve cartridge mounting differed from that on most haulers. On this particular design, the counterbalance valve is in the cylinder, not on a block at some distance from the telescopes where you would normally find it, with flexible hosing between. Most heavy hauling off-road dump trucks put the counterbalance valve amongst a group of valve elements somewhere else on the machine connected by flexible hoses.
As they’re getting ready to change out the cylinder, we asked. “Hey, what’s going to happen to the counterbalance valve in this one?” They said, “We’re sending the cylinder down to a cylinder repair shop in Montana and they’ll spend fifty grand reworking our cylinders, put in a new counterbalance, and ship them back up.”
We asked for the old counterbalance. We like to “steal” little parts of the machinery, take it home to our shop—that’s always fun— and discover what the counterbalance valve was going though. In our minds, it was worth investigating how they’ve actually managed to extrude or hydroform, swell the cylinder barrel a bit, with no hose blowouts.
I know people tell you that cartridges aren’t to be disassembled, but we take apart a lot of them anyway. Often it’s destructive disassembly, but that’s okay. The point was to get inside and find out what’s going on.
We spread out all the pieces, and what was most interesting is the main spool would not come out of the sleeve. It should just push out by hand, use your finger and it plops out on to the bench. But in this case, the spool would not come out of the sleeve. In fact, we put a wooden dowel at the bottom end and gave it a good couple of whacks with a hammer to eject it onto the bench. And we thought, “Well, that’s not right.” So, what did we find?
Interestingly, there were two bands of scuffing. We put the spool back in, with some effort, and after moving it back and forth a few times, in effect lapping the asperities, it was easier to work but still wasn’t flopping in and out. It didn’t take us long to find out that they’re dealing with the ingression of silica grains that are in the hydraulic fluid.
Well, the valve came out of the oil sands. So you can guess what type of contaminant they have there, silica grains that are really sharp. For all of the investigations that went on, it turned out to be a fairly simple cause. Sand is not supposed to be in the valves.
So let’s discuss pressure intensification issues between the blind side of the cylinder to the rod side. When you block flow in any way from fluid leaving the rod end, you get a multiplication of force from the blind end to the rod end, especially when the cylinders aren’t lifting a load. And of course with telescopic cylinders, they’re really more like a ram. You only have a very small angular piston surface area compared to a ginormous Frisbee disk sized piston on the blind side. I believe the ratio on this one was 50:1, so it wasn’t hard to imagine what pressures they were getting up to when the counterbalance valve on the rod side refused to open. It gets pretty interesting.
How the dirt got in is unknown. But I could tell you an interesting point about ingression, and how they fill these units. If you’ve been in mines, everything is done by quick couplers. And that sounds great, right? Except you rarely see any dust caps involved or a can of WD-40 and some kind of flushing.
Contaminant ingression isn’t fun. In this case the counterbalance valve that we’re playing with, mounted in cylinder, is nice from the point of view of making sure that you have a reasonable amount of load holding. A counterbalance valve is never quite the positive load-locking device that a check valve is. But it’s pretty good, and certainly adequate in this case.
In cylinder, it keeps you safe from the wonderful world of a blown hose and something coming crashing down on you. But on the other hand, it creates the scenario where if you should lose your load, which you do as the material slides up the back of the box and you go over center at the end of stroke, then the modulation aspect of the counterbalance valve is not available to you. Meaning that you are pinched down quite heavily during the over center aspect when you still had the load, but then once the load was gone the valve refuses to open up because of the sand granules in there. Then you’re just basically metering out; pinching the flow of oil leaving the rod end of the cylinder during extension and you’ve got that multiplication of pressure due to the multiplication factor of the surface areas.
The counterbalance valve should open, but instead, we’re into the wonderful world of intensified pressures. When the cylinder has no load to move, we basically just created an intensifier, as simple as that. So it was easy to explain in this case what they managed to do, and they had pressures well above 50,000 psi inside the cylinder, if I remember the calculations correctly. There were no hydraulic hoses to blow off because the counterbalance valve was contained in the cylinder.
We like to get out the microscope when these kinds of things happen, and use measurement software and see what can be learned. And look at how deep the scratches are. We found a good number of scratches that were a full ten microns wide. That would be an indication of the size of silica granules, maybe a little larger. When one checks into the clearances for these valves, you find out that if you had the ten and twenty micron sized chunks of silica in there, you’d definitely be larger than the clearance. And that easily explains why your valve is not opening.